Cómo dividir una cadena en caracteres de 2

Según el enunciado del problema, es necesario:

1. Cree una función similar a toUpper().
2. Maneje solo caracteres que no sean espacios.
3. Manejar caracteres impares que no sean espacios.
4. Formateo, es decir, agregar ', 'solo para el primer par que no sea

A continuación se muestra la muestra de salida del siguiente código

Enter the string: abraham lincoln
AB, RA, HA, ML, IN, CO, LN

Enter the string: abcde fghijk
AB, CD, EF, GH, IJ, K

Código:

#include <stdio.h>

char myToUpper(char c) {
    if(c >= 97 && c <= 122) {
        c = c-32;
    } //end if
    return c;
}

int main(void) {
    printf("Enter the string: ");
    char inputstring[50] = {0};
    scanf("%[^\n]", inputstring);

    char *p = inputstring;
    int counter=0;
    char firstChar, secondChar;
    int isFirstPair = 1; //this is to handle the flag for first pair, used as the counter for comma character (only first pair do not need comma)
    while (*p != 0) {
        //this is to change between state (1st and 2nd character), need to exclude space
        if (*p != ' ') { //only process non-space character, you can add more character filtering if needed
            if (counter == 0) { //first char
                firstChar = *p;
                counter=1;
            } else  { //if we are on the 2nd character, then capitalize it and print it (along with the comma if needed)
                secondChar = *p;

                if (isFirstPair == 1) {
                    isFirstPair = 0;
                } else {
                    printf(", ");
                } //end else

                printf("%c%c", myToUpper(firstChar), myToUpper(secondChar));
                counter=0;
            } //end else
        } //end if
        p++;
    } //end while

    //this is to handle when there are odd number of non-space characters
    if (counter  == 1) {
        if (isFirstPair == 1) { //this is for the case where there is only 1 character input
            printf("%c", myToUpper(firstChar));
        } else {
            printf(", %c", myToUpper(firstChar));
        } //end else
    } //end if

    return 0;
}